An aluminum base part is sometimes used for a handle holder in a motorcycle or a door knob and emblem in a passenger car. In this case, the aluminum base part becomes an exterior part exposed to a person's sight, and a good appearance of the aluminum base part is therefore desired.
Various techniques for improving the appearance of an aluminum base part have been proposed (see PCT Patent Publication No. WO 00143153 pamphlet (page 22) and Japanese Patent Laid-Open No. 2004-17738, for example).